The U.S. Energy Information Administration, EIA, estimates that nearly 500 trillion cubic feet of natural gas are recoverable in shale formations, like the Marcellusone that spans New York, Ohio, Pennsylvania and West Virginia, significantly boosting national gas reserves to what some estimate is a 100-plus-year supply.

An abundant supply of fuel is great, but modern gas-fired power plants also have lower operating costs than their antiquated coal-fired counterparts, folding in technological improvements that span not just years, but generations.

A November 2010 EIA report on power plant operating costs — the latest data available — found that a typical coal-fired plant costs $2,800 to $3,200/kilowatt of generation capacity, while a modern natural gas-fired plant costs around $1,000/kilowatt.

According to the most recent EIA figures, natural gas use in electricity generation rose by 40 percent year-over-year in March, while coal's market share fell by 20 percent.

While coal prices have also dropped by over 25 percent since April, experts say it won't affect the preference for natural gas.

The EIA says that of the 52 gigawatts (gw) of new power generation capacity to be added to the U.S. grid by 2015, half will come from natural gas. That's 10 times that of coal, which will fall into fourth place behind solar and wind power.

The ascension of natural gas has also led to a spate of coal-fired power plant closings.

In less than four years,Southern Company has cut its coal-fired generation capacity by half — going from 70 percent of its power fleet in 2008 to 32 percent today.

FirstEnergy announced six plant closures in January, while utility giant AEP and Duke Energy

also plan to shutter some coal plants.

In late May, AEP subsidiary Kentucky Power withdrew its $1 billion plan to upgrade one of its aging coal-fired plants in the heart of coal country, and didn't rule out repowering it with natural gas.

Of the nation's 600 coal-fired power plants — roughly 44 percent of U.S. power generation capacity — most are in the Midwest, with Ohio, Indiana, Pennsylvania and Illinois home to half of them. Some of those states also happen to be home to the Marcellus shale formation.

The attraction of natural gas comes at a time many coal-fired plants have reached the end of their life span.

According to DB Climate Advisors and the Electric Power Research Institute, an industry lab and think tank, nearly 60gw of coal-fired generation assets are antiquated, some of it up to 80 years old.

"They should have been put out of their misery long ago," says DB's Fulton.

The move to natural-gas powered electricity plants is not without obstacles.

Fulton says existing gas pipeline infrastructure, which brings gas to the plants, as well as the transmission infrastructure, which puts power into the national grid, is at a "near-term bottleneck."

Another issue is weak demand for power, whatever the cost of the fuel source, in an economy growing at a substandard rate.

"We have few plans for new generation at the moment," says Mark McCullough, executive vice president for generation at AEP.

However, a changed environmental landscape may favor natural gas.

According to the Department of Energy, emissions from natural gas plants can be up to 70-percent lower than coal-fired ones.

Still, some kind of legislation is inevitable, and Fulton says capital spending decisions by power plant developers must at least factor that in.

"Even if I don't care about climate change, someone does," he says, representing a view common among power plant developers facing the decision to switch from coal. "I might as well get on with it."

What's more, coming EPA regulation on mercury and particulates, as well as existing rules on acid rain-causing compounds — few of which are found in natural gas — are also negatives for coal.

McCullough says his company plans to add gas-fired generation, and that the confluence of abundant natural gas and lower plant operating costs have already paid off for his firm.

A significant portion of AEP's gas-fired generation was added over the past decade when other companies were forced to sell their plants at distressed prices.

"We bought them at pennies on the dollar, he says, adding that those units are running at high capacity now, given their relatively lower operating costs, due in large part to natural gas prices. "It appears we made a good decision."